Parasites including endoparasites and particularly ectoparasites such as lice, ticks, keds, flies, fleas, mites, scab, blowfly and itch mites commonly infect sheep and cattle resulting in poor quality, damaged wool and/or hides with serious consequences to the commercial viability of such primary industry. Such ectoparasites are typically controlled by the topical application of pesticidal formulations based on such active agents as synthetic pyrethroids and organophosphates. Such methods of topical application include jetting, plunging or dipping, or spotting or pouring-on.
Pour-on formulations of pesticides or ‘pour-ons’ as they are commonly known, have long been used to treat or prevent ectoparasitic infestation in domestic animals, particularly sheep, simply by locally topically applying a small amount of concentrated pesticidal formulation along the backline (the spine) of the animal generally in a single unbroken band extending from behind the ears to the withers. Alternatively, such pour-on formulations can be topically applied in discrete spots, generally around the neck and ears of the animal and also on the animal's back for example at the base of its spine.
Such formulations are typically applied by brushing or rolling on, by syringing or applying with a dosing gun, by using a commercially available applicator or pump dispenser, or are simply poured onto the back of the animal, typically between the base of the neck and the base of the tail.
The mode of efficacy of the ‘pour-on’ application method is based on the premise that the formulation, even though applied to a small area of the animal's external body surface, spreads around the animal's entire body surface, effectively controlling ectoparasites at sites on the animal distant from the area of localised application. Accordingly, many such ‘pour-on’ formulations are based on particular types of carriers which are said to enhance spread of the active agent around the animal's body. It has also been put forward as a hypothesis that the active is transmitted over the surface of the animal by diffusion through the hair or wool grease (in the case of a sheep). Alternatively, it has also been postulated that the pesticidal formulations work systemically by passing through the animal's skin into its bloodstream.
However, from recent studies, it has been noted that after application of a pour-on formulation to the backline of an animal, the formulation migrates poorly from the application site and therefore largely remains concentrated in a small area of the animal's body surface, that is mainly along its back. This results in the presence of localised high amounts of pesticide residues in the animal's wool or hair which can be toxic to handlers, and also in the costly practice of applying large volumes of the pour-on formulation in an attempt to achieve its uniform distribution around the animal's body. Additionally, many of the solvent systems used in such formulations can damage the hides and skins of the treated animals, particularly when localised in concentrated amounts. Such poor mobility has been observed in commercially available pesticidal formulations applied to the backline regardless of the solvent used to enhance the spread of the formulation. Such observations have led to the speculation that the mechanism operative in the case of ‘pour-ons’ applied to domestic animals is not a systemic mode and nor is it one in which the carrier system determines the degree of migration of the active about the animal. However, to date, no evidence as to an alternative mode of action of pour-ons has been put forward.